Bengaluru: Neanderthals and Homo Sapiens have interacted and mated over the years. But the details have been a mystery. A new study now identifies a pattern—most cross-species pairings were between Neanderthal men and Homo sapiens women.
According to a study by researchers from the Department of Genetics, University of Pennsylvania, Neanderthals and Homo sapiens had a preference for who they mated with. Published in Science on 26 February, it’s titled ‘Interbreeding between Neanderthals and modern humans was strongly sex biased’.
They write that it was not a single episode or a short window of contact, but rather a few thousand-year-long pattern of interaction.
“The Neanderthal DNA that persists in people today comes from an episode of interbreeding between 49,000 and 45,000 years ago, only a few thousand years before Neanderthals vanished,” reads an article in Science.
The interactions took place between modern humans who moved from the African subcontinent to Eurasia and the Neanderthals who had lived there for millennia.
While the researchers did not assign a reason to this preference—consensual or violent—Steven Churchill, a Duke University paleoanthropologist, speaking to Science, concludes the process was aggressive.
“It’s hard to reconcile that [males from one species monopolising females from the other] with anything but a competitive, unfriendly interaction.”
Also read: Homo sapiens and Neanderthals didn’t just share space and time. They interbred, exchanged ideas
The X chromosome
The study was motivated by a curious observation—the X chromosome in modern humans has a minimal amount of Neanderthal DNA.
The X chromosome is one of the sex chromosomes. Women have two X chromosomes, while men have an X and a Y chromosome.
“In contrast, autosomes—the non-sex chromosomes numbered 1 through 22—contain on average five times more Neanderthal DNA than the X chromosomes in people today,” writes Science.
There were two theories as to why the X chromosome lacked Neanderthal DNA: Imbalanced coupling of men and women with different genomes or an evolutionary accident.
The scientists at the University of Pennsylvania—Alexander Platt, Daniel N Harris and Sarah A Tishkoff —found that couplings between Neanderthals and Homo sapiens followed the same parental pattern over interbreeding events separated by 2,00,000 years. Lending credence to the first theory.
But such DNA provides “only half the picture,” said Platt to Science. That’s when he and his colleagues started probing the DNA that Neanderthals inherited from Homo sapiens.
When studying the genomes of a Neanderthal female who resided in Siberia roughly 1,22,000 years ago, they found evidence of earlier affairs between Neanderthals and Homo sapiens. When examining her X chromosomes, they found 1.6 times more Homo sapiens DNA.
This was the turning point for their research.
Also read: A massive genetic study tells us about Indians’ ancestry & perplexing presence of Neanderthals in it
A mating preference
The researchers stimulated various scenarios of modern humans migrating into Neanderthal territory and mating with them to understand this 1.6-fold excess.
But even in the most extreme scenario—where the migratory group was exclusively female—their models only showed a 1.3-fold excess of Homo Sapiens DNA on the X chromosome when compared to autosomes.
“What could most easily account for the 1.6-fold excess was a skewed mating preference: a scenario in which males with predominately Neanderthal ancestry and females with predominately modern human ancestry paired most often,” writes Science.
(Edited by Ratan Priya)